The present invention relates to a diaphragm for use in a master cylinder assembly of the type including a cylinder defining a pressure chamber and a reservoir port in communication with the pressure chamber, a piston mounted for reciprocal movement in the pressure chamber, and a reservoir in fluid communication with the pressure chamber of the cylinder. A diaphragm is typically employed in the reservoir to separate the reservoir into a liquid chamber in communication with the pressure chamber of the master cylinder and a gas chamber normally in communication with the atmosphere. The diaphragm forms an impervious barrier within the reservoir to keep liquid in the system and air out of the system; prevents moisture from entering the liquid; allows for expansion and contraction of the liquid under temperature variations; allows for variations in atmospheric conditions; and acts as a flexible medium to compensate for liquid flow into and out of the reservoir during actuation of the associated master cylinder and to compensate for changes in the volume of the liquid in the system due to wear in the system or system leakage.
Several forms of diaphragms have been used in the prior art.
One form of diaphragm in common usage has a generally top hat cross-sectional configuration. Whereas this diaphragm is satisfactory in some applications, it is capable of only a minor expansion relative to its total volume so that it is limited in its ability to compensate for under filling of the system, wear in the system, or leakage in the system.
Another form of diaphragm in common usage employs a pleated or bellows configuration. Whereas this type of diaphragm has a larger expansion capacity, it has a tendency to trap air between the pleats of the diaphragm as the diaphragm moves to a collapsed or contracted condition. This air can ultimately find its way into the master cylinder system. The pleated diaphragms are also subject to fatigue cracking at the crease points of the pleats.
Another form of diaphragm in popular usage includes a generally top had cross-sectional configuration with a rolled portion adjacent the upper end of the top hat. Whereas the rolled portion increases the capacity of the diaphragm as compared to the simple, top hat type diaphragm, the rolls tend to eventually fatigue at the crease points with consequent failure of the diaphragm.
Another form of prior art diaphragm includes a main body portion which, in a cross-section transverse to the central axis of the diaphragm, includes a plurality of radially extending circumferentially spaced arms arranged about the central axis. Whereas this arrangement provides a large expansion and contraction capacity, the arms have a tendency to become distorted or wrinkled during expansion and contraction with consequent derogation of the performance of the diaphragm and/or the wear life of the diaphragm.